1. Technical Field
The present invention relates to an exposure apparatus and to an image forming apparatus including the exposure apparatus.
2. Related Art
Printers employing an electrophotographic method generally include line printers (an image forming apparatus). The line printers include a charging unit, a line printer head (a line head), a developing device, a transfer device and the like, which are closely disposed on a circumferential surface of a photosensitive drum, i.e., a subject part to be exposed. That is, exposure is performed on the photosensitive drum, which is charged by the charging unit, by selectively emitting light from a light-emitting element provided in the printer head to form an electrostatic latent image. The latent image is then developed using the toner supplied from the developing device. The transfer device transfers the toner image onto the paper.
Inorganic or organic light emitting diodes (LED) of two columns, which are arranged in a zigzag pattern, are used as the light-emitting elements of the above-described printer head. However, it is very difficult to arrange several thousands of light-emitting points with high precision. For this reason, there has recently been proposed an image forming apparatus including a light-emitting element array, in which an organic electroluminescent (organic EL) device having light-emitting points formed with high precision is used as the light-emitting element, as the printer head.
However, the organic EL element has a very low luminance as compared with an inorganic LED, etc. In the line head using the organic EL element as the light-emitting element, it is very difficult to obtain the sufficient amount of light (luminance) required for exposure.
Under this background, a technique in which so-called “multiple exposure” is performed, in which one-line printing is executed by performing one-line exposure on the photosensitive drum using a plurality of light-emitting lines (e.g., see JP-A-2003-341140)
In the above-mentioned multiple exposure technique, however, when gray-scale levels are displayed, it is essential to perform multistage gray-scale level control per pixel (one line) in terms of its structure. Therefore, since a clock frequency for every line is twice or higher than the number of gray-scale levels, this exceeds the response speed of an active element such as a thin film transistor (TFT). As a result, a driver for driving each EL element cannot be internally mounted on the substrate. This necessitates an external mounting driver. Therefore, the degree of freedom not only in manufacturing the line head (printer head), but also in terms of the structure of an exposure apparatus or an image forming apparatus including such external mounting driver is limited.